Variability of the ocean carbon sink in MPI-ESM large ensemble simulations

The world oceans absorb large portion of anthropogenic carbon emissions and hence play a crucial role in modulating global carbon cycle and climate change. Previous studies based on observations revealed robust multi-year variations of oceanic carbon sink, i.e., regional decreasing trends were found in certain decades. This reflects the profound role of internal variability of the earth system relative to the direct response of ocean carbon sink to the increasing of anthropogenic carbon emissions. Earth system models, which include ocean biogeochemical components, are important tools for understanding the variability of ocean carbon cycle and its underlying mechanisms. The global mean ocean carbon uptake is relatively well captured by global models, but large uncertainties remain in reproducing regional and temporal variability. Yet, the contribution of internal model variability to the uncertainty in the oceanic carbon uptake remains unclear. By using a large ensemble simulation of (a) 100 ensemble members of historical and RCP4.5 simulation (1850-2035) and (b) 68 ensemble members of 1% per year CO2 increase up to 4xCO2 level simulation (155 years) based on the Max Planck Institute Earth System Model (MPI-ESM), we investigate the internal variability of the oceanic carbon uptake. Large internal variability of the oceanic carbon uptake is found among the ensemble simulations. By only slightly altering the initial states of the ensemble members, both increasing and decreasing 10-year trends are produced by the model in major carbon uptake regions. With the 100-member mean as a reference, we also test how many ensemble members are necessary to produce the forced trend. The Southern Ocean requires larger ensemble members to produce the forced signal probably due to its relative larger internal variability than the North Atlantic. The internal variability of ocean carbon uptake may increase in future high CO2 concentration world.